1. Field of the Invention
This invention relates to method and apparatus for sensing gases in air and more particularly to an array of metal oxide semiconductor gas sensors operated at controlled preselected temperatures to generate electrical responses which are processed by a microcomputer to accurately determine the respective concentrations of a plurality of gases in air.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,542,640 an array of nonselective semiconductor gas sensors are combined with a measurement and signal processing system to determine simultaneously and selectively the presence and concentration of several gases in ambient air. The method utilizes a set of nonlinear model equations, one equation for each gas sensor, which quantitatively relate the electrical response of each sensor to the concentration of the gases. Each model equation is characterized by a set of numerical parameters which describe the respective sensitivities of a particular sensor to each gas. Once the value of these parameters has been determined, gas detection is performed by a computation algorithm which transforms the measured responses of the sensors into a representation of the gas concentrations. The algorithm produces the value of gas concentrations most consistent with the model equations and with the measured sensor responses.
Due to the nonlinear nature of the model equations employed in U.S. Pat. No. 4,542,640 deviations of the sensor resistances from the values predicted by the model equations result in substantial inaccuracies in the measured gas concentrations. Such deviations can result from uncontrolled temperature fluctuations of the sensors, from slow changes over time of the sensor characteristics, or from the presence of unanticipated interference gases. Thus there is a need to provide a gas detection system that reliably and accurately detects several gases simulaneously in air in a compact and portable configuration that can be powered by conventional storage batteries.
The method for gas detection disclosed in U.S. Pat. No. 4,542,640 is known as deconvolution and is applicable to the detection of a particular set of target gases if one or more of the sensors responds to each gas. Also there must be at least as many sensors having distinct response characteristics as there are target gases. Deconvolution is also applicable in the case where gases other than the target gases are present, provided that none of the sensors respond appreciably to any of the interference gases. Thus there is a further need for a gas detection method and apparatus in which the sensitivities of each sensor to the respective gases can be selectively increased or decreased.
As disclosed in U.S. Pat. No. 4,457,161 an array of metal oxide semiconductor gas sensors fabricated by thick film printing processes are suitable for incorporation with a microcomputer in a gas detection instrument. The gas sensors formed on an insulating substrate are heated by an integral thick film heating element to an operating temperature in the range between about 400.degree. to 450.degree. C.
As disclosed in the publication entitled "A Microcomputer Controlled System For Characterizing Semiconductor Gases", Laboratory Practice, June 1983, Volume 32, No. 6 by B. Bott and T. A. Jones, a computer controlled system for characterizing semiconductor gas sensors employs a circuitry for maintaining a gas sensor at a programmable constant temperature. The circuit uses the out of balance signal of a Wheatstone bridge to maintain the resistance of the platinum heater, and hence the substrate temperature at a constant value. A particular resistor or resistor combination in conjunction with the circuitry regulates the operating point temperature of the semiconductor gas sensors. The temperature setpoints are distinct and are determined by the limited number of adjustable resistors, for example eight temperature setpoints would be available for a given temperature range. With this arrangement sensor operating temperatures can be maintained independent of ambient temperature or gas flow conditions. However, the discrete resistors and relays place certain limits on the size, cost, and reliability of the instrumentation.
Overall the accuracy of the known deconvolution method depends on the accuracy with which the model equations describe the actual gas sensor response. Because of the nonlinear nature of the gas response characteristics of semiconductor gas sensors, small deviations of the actual sensor responses from the predetermined model responses can produce significant errors in the deconvoluted gas concentrations. Such errors will not occur if the sensor response parameters remain constant and equal to the predetermined values. Because these parameters are temperature dependent, it is desirable to have a stable and reproducible temperature.
While it is known to simultaneously and selectively determine the presence and concentration of a number of target gases in ambient air and control the temperature at which the gas sensors operate, there is need to provide a computer operated temperature controller having the capability of an increased number of selectable setpoint temperatures.